Neutrino o’clock

To get to the bottom of one of the central questions in physics, a bit of interdisciplinarity can really give ideas a boost. In a paper recently published on AVS Quantum Science, experimentalist Federico Sánchez from the University of Geneva teamed up with theorist José Bernabeu and PhD student Alejandro Segarra from the University of Valencia in Spain and atom interferometry expert Dylan Sabulsky from CNRS’s SYRTE at the Paris Observatory. Together they developed a concept for an atomic clock-like experiment – a quantum potentiometer, essentially – that could not only measure the masses of the neutrinos present, but also distinguish the nature of the neutrinos. If it is built and if it works, many open questions about neutrinos would be answered.

“Our paper is like an open call to the community. First of all we’d like to present our idea and we’d like to get feedback from the many different communities it addresses – it’s important to draw on their expertise. To build it, we’d have to gather a whole new team of experts. But the good news is: it would not be very expensive,” says Federico Sánchez.

One problem their experiment wants to address is that of the exact mass of the neutrino. The neutrino mass is hard to measure with conventional methods – if there is such a thing as conventional in particle physics, because most detectors and experiments are quite extraordinary in their nature. Another open (but related) question is what is the mechanism to provide neutrinos with mass and whether they can be their own antiparticle.

Neutrinos come with a set of properties that present a set of challenges to whoever wants to study them. As per the underlying theory of elementary particles, the Standard Model, they should not have mass. However, experiments have delivered proof that they change from one flavour to another, proving that they do indeed have mass. To generate mass, the Higgs boson interacts with particles and their corresponding antiparticles. Fundamentally, we require two types of particles, and two corresponding antiparticles to realise this interaction. These particles have properties that physicists call “right-handed” and “left-handed” because they are similar to a spinning object. This said, however, neutrinos only have one of these handednesses for particles and one for antiparticles. In reality, other handedness- might exist, but only two out of the four interact with matter, so they are detectable, the others are inert or “sterile”. If they are are not interacting or “detectable”, do they exist? And, if not, how do neutrinos acquire their mass if they don’t interact with the Higgs boson?

The most accepted idea is that they are their own antiparticle, with identical properties except for their handedness: left-handed ones act as particles and right-handed ones as antiparticles. If neutrino and antineutrino were the same particle, they could be classified as Majorana particles, and currently the only way to test this hypothesis is using a mechanism called neutrinoless double beta decay in which two neutrons in a nuclei transform into two protons, releasing only two electrons. The standard neutrino, the one described in the Standard Model will not allow this radioactive decay. “There’s no alternative that we know now to this process, but it is complex and if we see no disintegration, it does not disproof the neutrino as Majorana particle. Not in all cases,” says Sanchez.

The tricky bit – one of many, as you may have gathered ­– is how to find out whether it is a neutrino or its antiparticle. At high energies both behave the same way and cannot be distinguished. You can only tell them apart at low energies and distances – when they become non-relativistic, i.e. they don’t follow the theory of relativity anymore.

A PhD student was charged with calculating what the interaction, the repulsive potential to be precise, would be between two nuclei through the exchange of two virtual neutrinos. On paper, the longer the range of the interaction the smaller the mass of the neutrino. So, by measuring the strength of this interaction at small physical distances (1 micron), it is possible to address the value of its mass. This force will also depend on the nature of the neutrino: is it the standard Dirac or the more exotic Majorana? So would it be possible to create an experimental environment in which all these needs are met? What is sensitive to energies as low as 10-56 Joules – 1016 times smaller than the gravitational attraction?

That’s when atomic clocks and atomic interferometry expert Dylan Sabulsky entered the equation. Atomic clocks use electronic transitions in atoms for timekeeping, making use of these very stable and precise microwave or optical resonance frequencies of atoms. Add a force induced by neutrinos to it and it could be quite a revolutionary approach! “It’s beautiful in theory,” Sánchez says. In practice it would look like this: starting with a cold atom source, you split the atomic wavefunction of the ensemble into two parts using a laser. The two parts form a coherent quantum mechanics state. One of the split parts passes through a hole in a micrometer scale cylinder, the source for the interaction, while the other part passes either through free space or a cylinder with a hole of a different diameter. The atoms close to the cylinder wall will see the neutrino potential while the others will not. This makes the atomic clocks ticking along each path at different speeds. When recombining the signal from both paths, we see that the clock of one of them went slower than the other one, creating an interference pattern.

“This idea requires the input of experts from many different fields, but the good news is: it wouldn’t be very expensive and its results would be groundbreaking for many other measurements. In fact, it’s so fundamental, it would be a groundbreaking experiment in itself,” says Sánchez. “I doubt I would see the result during my professional life, but we have to start at some point, right?”

Barbara Warmbein

The original paper “Neutrino mass and nature through its mediation in atomic clock interference”

Comment in AVS Quantum Science “Exploring the two-neutrino exchange potential”